The invention relates to a device on a draw unit for slivers, for example a draw frame, for determining adjustment values for the pre-draft, wherein a ratio of the circumferential speeds of center rolls and feed rolls of the draw unit can be changed in order to change the draft and wherein measured values can be recorded for a variable characterizing the draft forces in the main draft and/or pre-draft zones.
In such draw units, the pre-draft and main draft are sliver stretching operations in which the fibers of the slivers are more linearly oriented by stretching the slivers.
A known device is provided with a pressure bar for deflecting the sliver bundle in a pre-draft zone. A minimum displacement sensing element is assigned to the pressure bar and functions to generate a signal depending on the pressure exerted by the sliver bundle onto the pressure bar. The draft force is derived from the pressure exerted on the sliver bundle.
It is an object of the invention to create a draw unit of the aforementioned type for further improving, in particular, the adaptation of the draw unit to each assortment change and/or for quality changes in the produced fiber structure(s).
This and other objects of the invention are achieved by a device for determining and optimizing a pre-draft value on a draw unit for slivers. The draw unit has a feed roll pair, a center roll pair and a delivery roll pair, a pre-draft zone between the feed roll pair and the center roll pair, and a main draft zone between the center roll pair and the delivery roll pair. The device has a center roll drive motor for driving a lower roll of the center roll pair, a sensor for measuring values of a variable indicative of draft forces at the center roll drive motor, and means for controlling the drive motor in accordance with a function defining a relationship between the measured value and the pre-draft value. The function indicates an optimum region having at least one of an increase end point and an increase end region, the optimum region providing a parameter that is usable for optimizing the pre-draft value for the draw unit. A speed of the center roll drive motor is controlled to adjust a ratio of circumferential speeds of the center roll pair to the feed roll pair in order to change the pre-draft value.
A ratio of circumferential speeds of bottom center rolls to bottom feed rolls of the draw unit can be changed to change the draft (sliver stretching). Measuring values can be recorded for a variable characterizing the draft forces in the pre-draft zone and/or the main draft zone.
The measures according to the invention improve the adjustment of the draw unit considerably. Electrical signals from sensors permit an easy detection of undesirable deviations from the desired variables, for example machine-related and/or fiber-technological values, for each assortment change and/or during quality changes in the produced structure. In the process, the type and degree of the deviation is detected.
Undesirable deviations in the operation can be detected and used by the operators to adapt the draw unit by, for example, changing the nip line distances and/or the drafts (sliver stretching). According to the invention, a mathematical evaluation and a corresponding adaptation of the draw unit based on the evaluation results are possible. The evaluation can be carried out either by the operating personnel or can be carried out automatically by a computer together with the auto-leveler draw frame.